Spread-spectrum communication systems are well known and widely deployed. One class of spread-spectrum communication systems combines multiple antenna transmission techniques with advanced signal processing algorithms to create “smart antenna” using multi-input multi-output (MIMO) technology. MIMO technology may offer significant increases in data throughput and link range without additional bandwidth or increased transmit power by spreading the same total transmit power over the antennas to achieve an array gain that improves the spectral efficiency or to achieve a diversity gain that improves the link reliability. Because of its technical advantages, MIMO technology has become an important part of wireless communication standards such as IEEE 802.11n (Wi-Fi), the fourth generation mobile (4G), the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), worldwide interoperability for Microwave Access (WiMAX), and evolved high-speed packet access (HSPA+).
An N×M MIMO system utilizes N transmit antennas and M receiver antennas. The system may transmit up to N streams of data, each precoded with a length N precoding vector. At the receiver, a G-rake receiver may be used for demodulation transmitted streams. The Rake receiver derives its name from its rake-like structure, where multiple receiver “fingers” are used to receive multiple signal images in a received multipath signal. By coherently combining the finger outputs in a weighted Rake combiner, a conventional Rake receiver can use multipath reception to improve the Signal to Interference-plus-Noise Ratio (SINR) of the received signal. A Generalized Rake (G-Rake) receiver improves interference suppression performance over a conventional Rake receiver using more sophisticated generation of the combining weights.
A MIMO system utilizes pilots in its transmission of data streams. Three types of pilots are known in MIMO technology, and they are dedicated pilots, common pilots, and scheduled common pilots.
Dedicated pilots are precoded or beam-formed with data transmitted. Precoding may be done through coding with a vector and beam-forming may be performed through combining elements in a phased array so that signals at particular angles experience constructive combination while others experience destructive combination. Dedicated pilots may be used for precoded data demodulation when explicit precoder information is not included in control signaling. The advantage of dedicated pilot is that it can support high number of physical transmitting antennas.
Common pilots are pilots that are not precoded or beam-formed. Common pilots are generally used for 1) control channel demodulation, 2) non-precoded data demodulation, 3) downlink precoded data demodulation with explicit precoder information included in control signaling, and 4) channel quality indication (CQI) and MIMO channel measurement. Because channel estimation using common pilots does not need to be confined within a resource block, channel estimation performance can be improved with lower pilot density compared to dedicated pilots. Common pilots and dedicated pilots are not mutually exclusive. For example, in one MIMO system, common pilots may be used for CQI measurement and dedicated pilots may be used for MIMO channel measurement.
In addition, scheduled common pilots may be deployed in a MIMO system. Schedule common pilots are common pilots deployed in a branch only when the branch is utilized. For example, in a 4×4 MIMO system, common pilots may be used for CQI measurement. Additionally, for demodulation purposes, two additional common pilots are utilized whenever a four-branch user equipment (UE) is scheduled. That is, common pilots are used for CQI measurement while scheduled common pilots are used for data demodulation. The main objective of these additional pilots is to improve the performance of channel estimation (by transmitting at high power) while at the same time not transmitting these pilots all the time. At higher level, the scheduled pilots has the potential of achieving good performance of four-branch MIMO UE, while at the same time reduce the interference to other legacy users.
For demodulation, while prior art has explored inter-stream interference relating to multiple pilots in certain scenario within a 2×2 MIMO system, the accuracy of impairment covariance estimation has room for improvement with regard to common pilots. For 4×4 or higher dimension MIMO systems, there is no known effective and accurate estimation of interference caused by dedicated pilots and scheduled common pilots.